Heterogeneous catalysts are used, for example, in the preparation of hydrocyanic acid by the Andrussow process or in the preparation of nitric acid by the Ostwald process. In these reactions, the reactants and the catalyst are present in different phases and the reactions proceed at the surface of the catalyst.
In the case of noble metal catalysts composed of a permeable catalyst gauze, a fluid containing the unreacted starting materials flows through the catalyst gauze during the reaction. In general, the shaped catalyst body has one or more catalyst gauzes which are arranged one after the other and are arranged transverse to the flow direction of the fluid containing the starting materials to be reacted.
An important parameter of such catalyst gauzes is their catalytic effectiveness. Lastingly high conversions of the starting materials and good yields are achieved when the catalyst gauze has a large catalytically active surface, a low flow resistance and, at the same time, a high strength. Catalyst gauzes having a good catalytically active surface are frequently produced from noble metal wire using textile processing techniques, for example, by machine weaving, drawn-loop knitting or formed-loop knitting.
However, in these manufacturing methods, the flexural and tensile strengths and the ductility of the noble metal wires play a limiting role. Thus, for example, only noble metal wires having particularly wide diameters and tensile strengths are suitable for drawn-loop knitting of wires composed of, in particular, platinum-rhodium, platinum-palladium-rhodium, palladium-nickel, palladium-copper and palladium-nickel-copper alloys. As a result, the catalytically active surface area is fixed in a particular range.
Moreover, the catalyst gauzes produced by textile processing techniques have a high flexibility and a low stiffness because of their gas-permeable structure with measures and loops.
If a fluid flows through these catalyst gauzes, the gauzes are subjected to high pressures and thus high mechanical stresses. Thus, an ammonia-oxygen mixture is usually passed at high velocity through the catalyst gauze in the preparation of nitric acid by the Ostwald process, in order to achieve a high yield. In the Ostwald process, the reaction temperature is usually from about 800° C. to 1100° C. and the pressure is from 1 to 12 bar.
Catalyst gauzes having a high dimensional stability can, in principle, withstand high pressures better and contribute to uniform flow through the catalyst; a high stiffness and dimensional stability of the catalyst gauzes are therefore desirable, in principle, for reasons of reproducibility.
It is known that a higher strength of catalyst gauzes can be achieved when the gauzes have a secondary structure. U.S. Pat. No. 5,401,483 A and U.S. Pat. No. 6,030,594 A describe, for example, a secondary structure in the form of folding. A catalyst support having a plurality of knitted wire gauzes composed of metal with folding arranged one after the other is known from DE 23 53 640 A1. In the case of the catalyst support, the individual gauzes are also folded to effect mechanical stabilization.
However, folds provide secondary structures having a pronounced preferential direction, which can lead to formation of catalytically inefficient flow paths and locally different flow resistances, and thus to non-uniform flow behavior and a low efficiency and yield.
These disadvantages are avoided by catalyst gauzes having a secondary structure made up of three-dimensional embossed patterns as are known, for example, from U.S. Pat. No. 2,045,632 A and, in particular, from WO 93/24229 A1. The catalyst gauze of WO 93/24229 A1 is used for the catalytic conversion of ammonia into nitrogen oxide. The three-dimensional embossed pattern contains raised regions and depressions and is, for example, referred to as “embossed, contoured or provided with pits”. The secondary structure with which the gauze is provided serves to prevent deformation into a tortoise-shell-like structure, as is observed when planar gauzes are used, which reduces the flexibility of the gauzes. It is produced either directly during gauze manufacture or subsequently by pressing.
However, such catalyst gauzes are mechanically weakened in the region of the embossed places by deformation during the embossed process.